Construction machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy machinery use a hydraulic circuit to operate a variety of actuators and associated implements. During operation of the construction machines, there may occasionally arise a need for rapid operation of the actuators to move the implements back and forth (i.e. to shake the implement). For example, an implement such as a bucket is often used for excavating earthen materials, and due to the adhesive nature of the materials, an operator may need to shake the bucket to adequately remove material that is stuck to or remains within the bucket. An operator may also shake the bucket to break into hard ground. Such shaking can be manually accomplished by a rapid back and forth movement of a lever controlling a valve associated with the actuators of the bucket, or it can be accomplished by a controller that automatically and rapidly cycles the valve.
The bucket actuators are moved by a working fluid supplied from a variable displacement pump having load-sensing control. A problem associated with type of pump is that it can respond slowly to demand. That is, the variable displacement mechanism inside the pump may adjust slowly to sudden demands for increased flow. On the other hand, these mechanisms may quickly adjust to a low flow-producing state upon the sudden cessation of demand for flow. This delay ill transition to full flow and subsequent rapid return to a low flow condition may cause problems when an actuator or implement must be moved rapidly back and forth, such as when an operator needs to shake a bucket. In such a situation, the pump may have difficulty responding quickly to demands to rapidly extend, stop, and then retract the bucket actuator.
Attempts have been made to improve performance during a bucket shaking operation of hydraulic circuits having a load-sensing, variable-displacement pump. For example, U.S. Pat. No. 5,235,809 (the '809 patent), issued to Farrell on Aug. 17, 1993, discloses a hydraulic bucket shake circuit that allows rapid shaking of a bucket in response to an operator actuated switch. Upon activation of the switch, a directional control valve opens and sends a pilot pressure signal equivalent to a pump output pressure to a load-sensing input on an associated pump. This pilot pressure signal causes the load-sensing circuit to adjust the pump to maximum displacement, thereby allowing a rapid shaking of the bucket quicker than typical load-sensing pumps could otherwise respond.
Although the system disclosed in the '809 patent may allow a rapid initial extension of a bucket actuator, such as during a bucket shaking operation, it may have limited applicability. Specifically, the '809 system disables the variable displacement and load-sensing capabilities of the pump and, in doing so, turns the pump into a constant displacement pump displacing a maximum amount of fluid. This disabling may result in an inefficient system, because any excess fluid must be dumped directly to a tank, and the power used to pressurize the fluid is wasted. Additionally, the pressurized fluid being dumped across a relief valve to the tank may result in an undesired heating of the fluid, which may require a fluid cooling system, and/or prematurely degrade the quality and effectiveness of the fluid.
The disclosed hydraulic circuit is directed to overcoming one or more of the problems set forth above.